This invention relates to a thermal storage unit for use in a cooling process, and more particularly, to an improved ice-on-coil thermal storage apparatus and a method for utilizing ice which is formed and stored in a vessel during time periods when cooling demand is low.
During the summer months, cooling contributes approximately 35% of the electric demand in many areas of the United States. Unlike typical loads on the electric utility generating resources, cooling incurs a peak demand for only a few days or weeks each year. However, during these peak demand periods electric utilities often cannot meet the total demand, and many have adopted usage and demand sensitive billing systems. In this type of system, electricity rates are more expensive during the hottest hours of the day when demand is the greatest than in the evening hours. In some case, the high daily usage period is used to determine the rate structure for the facility throughout the year, allowing the summer peak usage to inflate a facility's rate all year, not just during the summer. During recent years, the interest in thermal storage has increased based on the soaring cost of electricity, which has encouraged additional energy conservation measures. Many utilities have offered incentive programs and special rate structures to encourage cool storage use. In addition, cool storage technology has improved significantly since 1980, and designers and their clients continue to express a strong interest in the use of cool storage to reduce energy costs. This is due in part to the fact that by flattening out the demand curve for power, the power companies can improve their operating efficiency and avoid the construction of new generating plants to meet peak demands.
Several types of thermal storage are in use today. Chilled water storage is known and utilizes a large tank or reservoir of liquid that gets chilled at night by conventional air conditioning chillers. During the day, the water is circulated through the building or process that needs cooling. The water picks up heat at the load and is returned to the tank. The chilled water storage tanks can easily be multi-million gallon tanks, and in some instances the tank size can easily be larger than the building that it is cooling.
Ice storage is used since ice is able to storage more thermal energy (Btu's) in a much denser form. Ice can be used because of its latent heat content to store seven to ten times more Btu's per cubic foot than chilled water, which reduces the storage area or vessel size. One known method utilizes ice harvesting, in which an ice machine continuously makes cubes, slush or sheets of ice. The ice is stored in a tank, similar to the chilled water tank, but much smaller. As cooling is needed during the day, ice water can be drawn from the tank and pumped to the heat load. The warm water that returns from the load goes back to the ice tank to be cooled again. One problem with this type of ice thermal storage is the formation of large icebergs from the cubes, slush or sheets of ice in the tank that can be difficult to melt and which reduce the available surface area of the ice to absorb the heat load.
One method of preventing icebergs in the tank is to fill the tank with a multitude of sealed containers full of water. The containers can be loosely piled in the tank or neatly stacked such that they fill the tank and still have passages for a cooling medium to flow around them (usually a glycol anti-freeze solution). Air conditioning chillers modified to cool the cooling medium to sub-freezing temperatures are connected to the tank. As the cooling medium is circulated through the tank, the water in the containers freezes solid. For melt-out, the same cooling medium is circulated through the tank and to the heat load that needs cooling. However, due to the proprietary container designs needed for such systems, encapsulated ice systems can be very expensive. Additionally, the large volume of anti-freeze cooling medium can also add to the expense of such systems.
Ice-on-coil systems for thermal storage are also known. In these systems, a series of coils with widely spaced apart tubes is submerged in a tank of water. Cold (sub-freezing) anti-freeze solution is pumped from a specially modified chiller through the coil tubes during the ice build-up cycle. Ice forms as concentric cylinders on the surface of the tubes and can be built to a thickness of one to several inches. Using the geometry of the coil (careful tube placement) the ice build up can be kept very uniform. The ice is maintained in its proper place, and no icebergs are formed such that a maximum amount of ice surface area is exposed to the tank water at all times. The surface area of exposure is important to obtain maximum cooling during the melt out cycle.
In one known apparatus, the thermal storage equipment includes a cooling coil which carries a liquid refrigerant, such as brine or ethylene glycol solution, through a pool of freezable storage liquid, such as water. The pool of water, or the like, is confined within a vessel and the refrigerant coil, usually in the form of tubing bent into a serpentine path with a plurality of tube runs, is immersed in the pool. The refrigerant tubes are generally stacked in parallel within the pool and connected between inlet and outlet headers which receive and discharge the refrigerant liquid from and to one or more heat exchangers in which the refrigerant liquid is cooled during the ice storage cycle, and warmed during the cold supply cycle. The storage liquid is usually agitated during at least certain periods of operation to lessen temperature stratification.
There are two ways of extracting the cooling capacity from the stored ice; internal melt and external melt. With internal melt systems, the anti-freeze solution inside the coils is pumped to the cooling load where it picks up heat. When it returns to the coils, the warmed glycol is cooled by the melting ice on the tubes as the ice melts from the inside-out. External melt systems circulate the storage liquid from the tank to the heat load and return it to the tank to melt the ice off the tubes from the outside-in during periods of high demand on the cooling system.
A common enhancement to the performance of ice-on-coil systems during melt out is to agitate the tank water to expose as much tank water to as much ice surface area as possible. This provides cooling water at low temperatures, close to 32.degree. F. A popular method of tank agitation is to use an air bubbler system. Low pressure air introduced to the bottom of the tank under the coils produces bubbles that rise up through coils and generate vertical currents of water. This agitation is desired because the typical circulation rate of tank water to and from the heat load is quite small compared to the volume of the tank and provides insignificant agitation to a large tank. Tank water agitation is useful on both external and internal melt systems. For internal melt systems, agitation is not much help during the initial part of the melt. However, after the initial phase of the melt cycle, the warmed melted ice (water) surrounding the tubes begins to break out through the ice cylinders. At that point, the ice will be melting internally and externally, and it is at that point agitation of the tank water improves the cooling process.
It is also known to provide thermal storage units in which the coils are frozen solid with ice during the build up cycle. While this provides a maximum amount of thermal storage in a specified volume, such systems are only useful for internal melt, and melt performance is hampered by a lack of agitation.
In order to increase the heat transfer efficiency between the ice and water circulated in the tank, it is known in the art to form ice on coils which are arranged as widely spaced serpentine tube circuits, connected by manifolds to inlet and outlet headers. Such systems typically use steel tubes ranging in size from 1/2 inch to 13/4 inches in diameter, and ice thickness can range from 1/2 inch to 21/2 inches. In the known ice-on-coil systems, as shown in FIG. 1, which illustrates a portion of a coil through two serpentine circuits, the serpentine circuits consist of generally horizontal tubes 3 with 180.degree. bends on the ends, with the tubes being arranged in generally vertically aligned rows which are horizontally spaced apart from adjacent circuits. In the known system of FIG. 1, the tubes 3 have an outside diameter of 1.05 inches, and are horizontally located apart by a dimension X of 37/8 inches on center and vertically located apart by a dimension Y of 37/8 inches on center. Ice envelopes 4 are formed to a thickness of approximately 1.4 inches such that the diameter of the ice envelopes 4 formed on each tube 3 is approximately 37/8 inches.
In some cases, the tubes 3 are staggered vertically in a nesting effect to improve or create a clearance space or gap between the ice cylinders 4, as shown in FIG. 2. Using the same center-to-center dimensions X and Y and the same dimensions for the tube 3 and thickness of the ice 4 in FIG. 2 as in FIG. 1 results in a clearance gap of approximately 0.43 inches between the round cylinders of ice 4 in adjacent rows. Alternatively, this space is used to allow a certain amount of ice overbuild into the clearance gap. However, it is easy to see that any excessive overbuild will result in horizontal bridging which will quickly close out the clearance passages necessary for agitation and very quickly will reduce the surface area of the ice which is available for cooling. Once horizontal bridging occurs, melt out performance deteriorates and the large remaining icebergs become quite difficult to melt out.
Clearance spaces, while necessary, detract from packing efficiency of the coil. Packing efficiency is the ratio of the volume of ice actually formed and stored in comparison to the available space for frozen storage liquid around the coil assembly (excluding necessary clearance spaces designed for agitation). Since the cross-sections of the tubes and ice envelopes are generally constant, packing efficiencies can be calculated based on unit cross-sectional areas of the ice envelopes and the available area for frozen storage liquid to form. In systems in which horizontal bridging is not desired in order to allow agitation for more efficient melt out of the ice, the clearance gaps detract from the overall packing efficiency, since the vertical passageways remain unfrozen. Packing efficiency is maximized by filling the space around the tubes of the coil as fully as possible with ice, without leaving unfrozen areas within the confines of the potentially freezable areas. As shown in FIGS. 1 and 2 of the known systems, round cylinders of ice 4 formed on the tubes 3 of the cooling coil leave significant open spaces in the available space for frozen storage liquid around the coil assembly. In FIGS. 1 and 2, the ice packing efficiency is approximately 0.785 (.pi./4), since the potentially freezable area would be a square unit area which encompasses a cross-section of the cylindrical ice 4. This is about as efficient packing as possible with the current art shown in FIGS. 1 and 2 without the risk of overbuilding which would result in horizontal bridging and detract from cooling efficiency during melt out.
It is possible to reduce the vertical spacing between the tubes to form a smaller vertical gap between adjacent tube surfaces and allow a small amount of bridging in the vertical plane, as shown in FIG. 3, where Y is less than 37/8 inches and the remaining dimensions are the same as in FIGS. 1 and 2. When this is done, the packing efficiency can be greater than 0.785. It is known to reduce this vertical spacing in order to encourage bridging in the vertical direction while still leaving horizontal gaps to leave vertical passageways and prevent horizontal bridging which reduces overall cooling efficiency. The result of this vertical bridging is to form vertical slabs of ice with vertically oriented clearance spaces between the vertical rows of tubes for tank water circulation. The generally vertical orientation of the spaces is important for the bubbler agitation to work.
In calculating packing efficiency, it is important to note that if the quantity of ice is calculated based upon ice thickness, then the areas of overlap 5 due to vertical bridging would be counted twice (once for ice around the upper tube in the overlap area 5 and once for ice around the lower tube in the overlap area 5), indicating a deceptively high amount of ice in storage. However, in fact, what happens is that the ice tends to build up in the areas of intersection 6 adjacent the overlap areas 5 and it has been shown to make up for the double counted areas. See FIG. 3. For the purposes of this disclosure, packing efficiency has been calculated without counting the overlap areas twice. Additionally, the overbuild of ice in the areas of intersections was not counted since it is difficult to predict, and the agitation space between adjacent vertical rows has not been included. As a result, all packing efficiency calculations herein for a single vertical row are conservative, and the effects in actual practice will be greater than shown here. Of course, putting the tubes of the coil closer together vertically has limits. Eventually, the increases in efficiency become more and more costly. When additional tubes are added, they take up additional space in the coil that could be occupied by ice. There is also a limited ability of manufacturers to make smaller, tighter return bends for connecting the tubes together.
All of the known ice-on coil thermal storage units utilize tubes with a round cross-section. The present invention provides for higher packing efficiency without undesired horizontal bridging in order to provide an improved thermal storage apparatus.